Refrigerator interior liner

ABSTRACT

A rubber modified monovinylaromatic polymer composition may include a monovinylaromatic polymer matrix. The monovinylaromatic polymer matrix may have an average molecular weight in weight  Mw  above 150,000 g/mol. Rubber particles may be dispersed in the monovinylaromatic polymer matrix. The rubber particles may have a volume median particle size (RPS volume) ranging from 7 μm to 10 μm, a monomodal distribution, a ratio of RPS volume to surface median particle size (RPS surface) that is below 2, a crosslinking of the rubber expressed as a swell index (SI) above 13.8, and a rubber phase volume fraction (RPVF) of at least 39%. A refrigerator interior liner may be made of the rubber modified monovinylaromatic polymer composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.14/222,856, filed on Mar. 24, 2014, which is a Divisional of U.S. patentapplication Ser. No. 13/203,886, filed on Nov. 11, 2011, now issued asU.S. Pat. No. 8,785,545, which is a National Stage Entry ofPCT/EP2010/052984, filed on Mar. 9, 2010, which claims priority from EP09154704.2, filed on Mar. 10, 2009.

FIELD OF THE INVENTION

The present invention concerns a refrigerator interior liner made of arubber modified monovinylaromatic polymer composition such as, by way ofexample, an ABS (acrylonitrile-butadiene-styrene) or a HiPS (high impactpolystyrene). “Refrigerator”, in the present invention has to beunderstood as (i) an apparatus having a compartment at about 4° C. and acompartment at −18° C. or even as low as about −30° C. or as (ii) anapparatus (freezer) having one or more compartments at −18° C. or evenas low as about −30° C.

BACKGROUND OF THE INVENTION

The liner of a refrigerator is in contact on one side with the foodinside the refrigerator and on the other side with the insulation foam.Said insulation foam, typically a polyurethane foam, is made with ablowing agent which can cause environmental stress cracking (ESCR) ofthe liner. ESCR is the formation of cracks in a material caused byrelatively low tensile stress and environmental conditions. The blowingagent can cause liner blistering, catastrophic cracks, tiny cracks(crazing) and loss of impact properties (embrittlement), as well asstress whitening and/or dissolution. The following prior arts relate tothis subject.

U.S. Pat. No. 5,221,136 describes a refrigerator comprising an ABS orHiPS liner but said liner comprises a barrier layer on the side facingthe polyurethane foam. The barrier layer comprises a polymer orcopolymer of ethylene or propylene containing 0 to 40% by weight of ablock copolymer rubber.

U.S. Pat. No. 5,834,126 is similar to the one above but the compositionof the barrier layer resistant to the action of polyurethane foamblowing agents has an effective amount of a polyethylene modified with acompound such as maleic anhydride, maleic acid, malic anhydridederivatives, maleic acid derivatives, or mixtures thereof and aneffective amount of a rubber. The barrier layer composition may containpolyethylene, polypropylene, polybutylene, or copolymers thereof.

Other prior arts are dealing with rubber modified monovinylaromaticpolymers which are deemed to be of interest in a lot of applicationsincluding the refrigerators but are not specific to the refrigerators.

U.S. Pat. No. 6,706,814 describes a rubber modified monovinylidenearomatic polymer comprising:

a) a monovinylidene aromatic polymer matrix; and

b) rubber particles dispersed therein, characterized in that the rubberparticles are produced from a diene rubber having substantially linearstructure containing less than one long chain branch per 10,000 carbonatoms in the polymer backbone with a solution viscosity of 5 cPoise to1,000 cPoise and a Mooney Viscosity of 5 to 120.

The rubber particles are dispersed in the form of small and largeparticles, wherein the volume average particle diameter of the smallparticles is from about 0.1 to about 2 micrometers and volume averageparticle diameter of the large particles is from about 2 to about 6micrometers. A lot of applications are described including refrigeratorsand freezers.

U.S. Pat. No. 6,545,090 describes a rubber modified monovinylidenearomatic polymer comprising:

a) a monovinylidene aromatic polymer matrix,

b) rubber particles dispersed therein, characterized in that the rubberparticles are produced from a diene rubber having I) a high molecularweight component and II) a low molecular weight component; the highmolecular weight component having a weight average molecular weight atleast two and one half times greater than the weight average molecularweight of the low molecular weight component and the low molecularweight component constitutes from about 20 to about 80 weight percent ofthe total rubber content, wherein both components I and II have a 1,4cis content of greater than 70 percent and III) the rubber is graftedusing a graft promoting chemical initiator, with monovinylidene aromaticpolymer to the extent that the amount of grafted rubber is at least 30percent of the total rubber at phase inversion.

The rubber particles are dispersed in the form of small and largeparticles, wherein the volume average particle diameter of the smallparticles is from about 0.1 to about 2 micrometers and the volumeaverage particle diameter of the large particles is from about 2 toabout 6 micrometers and the small rubber particles are from 20 to 80weight percent of the total rubber. A lot of applications are describedincluding refrigerators and freezers.

U.S. Pat. No. 6,441,090 describes a rubber modified monovinylidenearomatic polymer having a bimodal particle size distribution comprising:

a) rubber particles of a star or branched low viscosity rubber having avolume average particle size of from 0.1 to 2 μm, and a cellular or coreshell morphology or mixture thereof, and

b) rubber particles of a star or branched low viscosity rubber, lineardiene rubber or block copolymer rubber having a volume average particlesize of from 0.5 to 10μ,

characterized in that the rubber particles of b) are more dense than therubber particles of a), having a smaller occluded monovinylidenearomatic polymer content than the particles of a), wherein the particlesof a) are from 50 to 99 weight percent of the total diene rubbercontent. These rubber-reinforced bimodal compositions are described asuseful in a wide variety of applications such as consumer electronics,small household appliances, toys and furniture. These polymers are alsodeemed to be useful in extrusion applications such as in the preparationof a gloss layer using coextrusion techniques for refrigerator liners.

U.S. Pat. No. 7,115,684 describes a mass polymerized rubber-modifiedpolymeric composition comprising: a continuous matrix phase comprising apolymer of a monovinylidene aromatic monomer, and optionally, anethylenically unsaturated nitrile monomer, and discrete rubber particlesdispersed in said matrix, said rubber particles produced from a rubbercomponent comprising from 5 to 100 weight percent of a functionalizeddiene rubber having at least one functional group per rubber moleculecapable of enabling controlled radical polymerization; wherein thecomposition is further characterized by:

a) a volume average rubber particle size of from about 0.15 to 0.35micron,

b) a total rubber phase volume between 12 and 45 percent, based on thetotal volume of the combination of the matrix phase and the rubberparticles;

c) a partial rubber phase volume between 2 and 20 percent characterizedby rubber particles having a volume average particle size of greaterthan 0.40 microns; and

d) a crosslinked rubber fraction of at least 85 percent by weight, basedon the total weight of the rubber particles.

These rubber modified polymers can be used in a variety of applicationsincluding injection molding and thermoforming of refrigerator liners,household appliances, toys, automotive applications and furniture.

Another technical problem is the environmental stress crackingresistance of the plastic material induced by localized sharptemperature variations and/or the presence of fats and oils in food orany aggressive chemical agent that may get in contact with the plasticmaterial.

It is proposed to provide a plastic sheet structure to be thermoformedinto a refrigeration liner that is resistant to chemical attack.

It is an object of the invention to provide a refrigeration applianceliner to be fabricated from a thermoformable, plastic sheet materialexhibiting resistance to chemical attack e.g. blistering, cracking,crazing, as mentioned above, by polyurethane foaming agents.

It is an object of the invention to provide a refrigeration applianceliner to be fabricated from a thermoformable, plastic sheet materialwhich retains a high level of toughness (impact properties) and strength(tensile properties), even at low temperatures (−20° C. or less).

It is another object of the invention to provide a liner made from aplastic sheet material that maintains processability similar to HIPS orABS, including favorable extrusion conditions and similar thermoformingbehavior.

It has now been discovered a refrigerator interior liner made of arubber modified monovinylaromatic polymer composition which at leastfits one of the above criteria. In brief the main features of thispolymer composition are (i) a large size of moderately crosslinkedrubber particles combined with a monomodal particle size distributionand (ii) a rubber phase volume fraction (RPVF) of at least 39%. Thebroadness of the rubber particle size distribution estimated by the RPSvolume-to-RPS surface ratio, is preferably below 2.0, more preferablybelow 1.5 and most preferably equal or below 1.4. RPS volume means thevolume median particle size, RPS surface means the surface medianparticle size of the rubber.

Secondarily, the average molecular-weight in weight of the PS phaseshould be sufficiently high, whereas the global concentration oflow-molecular weight plasticizers such as white mineral oil and PSoligomers should be kept low-to-moderate.

The present invention also relates to a process to make said polymer.The process for making HIPS is well known to those skilled in the artand consists of polymerizing styrene monomer in the presence ofdissolved rubber. Polymerization of styrene, and optionally a comonomer,is initiated by heating and/or by an initiator, by way of example aradical initiator. The rubber is “dissolved” in the styrene monomer(actually the rubber is infinitely swollen with the monomer). The usualrubber types utilized in the manufacture of HIPS include polybutadiene(PB), styrene-butadiene rubber (SBR), and styrene-butadiene-styrenerubber (SBS). Polystyrene is initially formed from the styrene monomerwithin the homogeneous rubber solution in styrene. At the beginning ofthe polymerization the reacting solution is at a point prior to therubber/styrene inversion point, i.e. the point at which the solutionbeing reacted goes from polystyrene particles in a rubber/styrenemonomer matrix to rubber particles in a polystyrene matrix. When thedegree of polymerization is about equal to the weight % of rubber in thesystem, it inverts e.g. the styrene/styrene polymer phase becomescontinuous and the rubber phase becomes discontinuous. Styrene ispolymerized around and within the rubber particles which leads topolystyrene inclusions in the rubber particles. A portion of the styreneis polymerized by grafting on the rubber, another portion ishomopolymerized, said portion is referred to as a “non-grafting”polymerization. In HIPS a part of the styrene may be replaced byunsaturated monomers copolymerizable with styrene such as othermonovinylaromatic monomers, alkyl esters of acrylic or methacrylic acidand acrylonitrile. The same mechanism of “grafting” and “non-grafting”occurs with the styrene comonomer, which means one portion of thestyrene and of the comonomer are polymerized by grafting on the rubber,another portion of the styrene and of the comonomer are copolymerized.The properties of HIPS are related to the amount of rubber, the type ofrubber, the rubber particles size distribution and volume fraction aswell as the polystyrene included in the rubber particles. The proportionof styrene, and the optional comonomer, which is grafted (polymerized bythe “grafting” way) is linked to the functionalization of the rubber.Rubber-modified vinylaromatic polymer compositions are well-known in theprior art. Composition fine-tuning so as to reach well balanced physicalproperties remains however a matter of know-how. Apart from the controlof the phase rheological behaviours, the control of rubber phasegrafting that occurs typically in situ during the conventional radicalHIPS process is a challenge hard to overcome. Rubber phase grafting isindeed usually adjusted through the addition of organic peroxides,generating preferably H-abstracting radicals by thermal decomposition,and well-chosen according to their half-life decomposition temperatureand the reactor temperature settings. However, the in situ rubbergrafting during the HIPS process remains intrinsically a randomreaction.

The main features of the process of the present invention are the use ofa grafting initiator, a viscous rubber essentially of linear structureand the use of a chain transfer agent before the phase inversion.Additionally, the process as described in the present invention isperformed in one or several polymerization reactors, under batch-wise orcontinuous polymerization conditions, with preferably limitedback-mixing conditions within the inversion reactor—i.e. the reactorwithin phase inversion takes place—and within the just-former andjust-subsequent reactors, and with also limited spillback flows betweenthese aforementioned reactors.

EP 1201693 A2 has already described similar compositions for foodcontainers and trays for a refrigerator but not for the liner. Moreoverthis prior art is silent on the ESCR of the liner due to the blowingagent of the insulation.

WO 94 12551 A1 describes impact styrenic polymers having good ESCRproperties and improved physical properties to be employed in theproduction of thinner sheet stock for use in the manufacture of, forexample, refrigerator liners, thereby resulting in reduced linermanufacturing costs. The compositions have a volume average particlesize of at least 4 microns and a melt strength of at least 4.5 grams.There are no specific requirements for the rubber and nothing ismentioned on the broadness of the rubber particle size distribution.

BRIEF SUMMARY OF THE INVENTION

The present invention is a refrigerator interior liner made of a rubbermodified monovinylaromatic polymer composition comprising:

a) a monovinylaromatic polymer matrix having an average molecular weightin weight Mw above 150,000 g/mol,

b) rubber particles dispersed therein, said particles having:

a volume median particle size (RPS volume) of about 8.5 μm,

a monomodal distribution with essentially no shoulder,

a low-to-moderate crosslinking of the rubber expressed as a swell index(SI) above 13.8,

a rubber phase volume fraction (RPVF) of at least 39%,

c) a moderate amount of low-Mw plasticizers, defined as the weightfraction of solubles in methanol, and such that the weight ratioc/(a+b+c) ranges from 0 to 5%.

The present invention is also a process for preparing the abovecomposition to make the refrigerator interior liner comprising:

a) forming a polymerizable mixture comprising at least onemonovinylaromatic monomer and optionally one or more comonomers,

b) dissolving at least a viscous rubber of essentially linear structurein said polymerizable mixture to form a rubber containing polymerizablesolution,

c) contacting a free radical initiator and a chain transfer agent withthe polymerizable mixture at conditions whereby phase inversionsubsequently occurs, wherein the chain transfer agent is capable toproduce an increase of the rubber to PS phase viscosity ratio within theinversion reactor,d) continuing the polymerization of the solution obtained at step c),optionally in the presence of a free radical initiator, until amonovinylaromatic polymer matrix having rubber particles dispersedtherein is obtained.

Optionally there is an additional step for degassing the product of stepd) to separate the optional unpolymerized monomers and comonomers,optional diluents and recovering the high impact monovinylaromaticpolymer.

Advantageously the chain transfer agent makes the rubber particles lesssensitive to the shear rate within the inversion reactor. This resultsin a homogeneous distribution of large rubber particles.

The advantageous rubber of essentially linear structure within the frameof the present invention is characterized by SV-to-Mooney ratio of atleast 2.8, and more preferably above 3.3.

Advantageously the Mw of the rubber ranges from 100,000 to 500,000 andpreferably from 280,000 to 360,000 g/mol. Advantageously thepolydispersity index of the rubber ranges from 2.1 to 2.5 and preferablyfrom 2.1 to 2.3. Rubber molecular-weights can be measured by theconventional size-exclusion chromatography techniques. They are hereexpressed in PS equivalents, i.e. using iso-molecular PS samples ascalibration standards.

The rubbers particularly suitable for this invention have a solutionviscosity (SV), measured at 5.43% weight in toluene or styrene, of 50 to1000 centipoises, preferably from 100 to 500 centipoises and morepreferably from 120 to 250 centipoises. The rubbers particularlysuitable for this invention also have a Mooney viscosity (ML4+1, 100°C.) of 5 to 120, preferably from 10 to 100 and more preferably from 30to 60.

The melt-flow index of the modified monovinylaromatic polymercomposition to make the liner, measured following ISO 1133 (5 kgloading, 200° C.), is advantageously from 2 to 10, more advantageouslyfrom 2 to 7, preferably from 2.5 to 6 and more preferably from 2.5 to 5.

The present invention is also a process for preparing a high impactmonovinylaromatic polymer comprising:

a) forming a polymerizable mixture comprising at least onemonovinylaromatic monomer and optionally one or more comonomers,

b) dissolving at least a viscous rubber of essentially linear structurein said polymerizable mixture to form a rubber containing polymerizablesolution,

c) contacting a free radical initiator and a chain transfer agent withthe polymerizable mixture at conditions whereby phase inversionsubsequently occurs, wherein the chain transfer agent is capable toproduce an increase of the rubber to PS phase viscosity ratio within theinversion reactor,d) continuing the polymerization of the solution obtained at step c),optionally in the presence of a free radical initiator, until amonovinylaromatic polymer matrix having rubber particles dispersedtherein is obtained.

Optionally there is an additional step for degassing the product of stepd) to separate the optional unpolymerized monomers and comonomers,optional diluents and recovering the high impact monovinylaromaticpolymer.

Advantageously the chain transfer agent makes the rubber particles lesssensitive to the shear rate within the inversion reactor. This resultsin a homogeneous distribution of large rubber particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts injected halters exposed to fatty esters and submitted todeformation.

FIG. 2 depicts RPS distributions in volume of products.

FIG. 3 depicts a representation of ESCR resistance with time, with oliveoil as the stress-cracking agent.

DETAILED DESCRIPTION OF THE INVENTION

Monovilylaromatic polymer suitable for the present invention are thoseproduced by polymerizing a vinyl aromatic monomer. As regards themonovinylaromatic monomer, it relates to any aromatic having a vinylfunction. By way of example mention may be made of styrene, vinyltoluene, alpha-methylstyrene, alpha-ethylstyrene, methyl-4-styrene,methyl-3-styrene, methoxy-4-styrene, hydroxymethyl-2-styrene,ethyl-4-styrene, ethoxy-4-styrene, dimethyl-3,4-styrene,chloro-2-styrene, chloro-3-styrene, chloro-4-methyl-3-styrene,tert.-butyl-3-styrene, dichloro-2,4-styrene, dichloro-2,6-styrene,vinyl-1-naphtalene and vinylanthracene. It would not depart from thescope of the invention to use more than one monovinylaromatic monomer. Apart of the monovinylaromatic monomer may be replaced by unsaturatedmonomers copolymerizable with styrene. By way of example mention may bemade of alkyl esters of acrylic or methacrylc acid, acrylonitrile andmethacrylonitrile. The proportion of comonomer may be from 0 to 50% byweight for respectively 100 to 50% of the monovinylaromatic monomer.

In a specific embodiment the monovinylaromatic polymer comprises:

i) from 60 to 100 weight % of one or more C₈₋₁₂ monovinylaromaticmonomers; and

ii) from 0 to 40 weight % of one or more monomers selected from thegroup consisting of C₁₋₄ alkyl esters of acrylic or methacrylc acid andacrylonitrile and methacrylonitrile; which polymer may contain fromabout 2 to about 20 percent, preferably from about 3 to about 17percent, more preferably about 3 to about 15 weight percent rubber,based on the total weight of the rubber modified monovinylaromaticpolymer.

Advantageously Mw ranges from 150,000 to 250,000 g/mol, preferably from160,000 to 190,000 g/mol and more preferably from 160,000 to 170,000g/mol. The polydispersity index (ratio Mw/Mn) ranges e.g. from 2.2 to 3,advantageously from 2.4 to 2.7.

In an embodiment Mw ranges from 150,000 to 170,000 g/mol, preferablyfrom 150,000 to 160,000 g/mol.

In another embodiment Mw ranges from 180,000 to 220,000 g/mol,preferably from 190,000 to 210,000 g/mol.

As regards the rubbers preferably employed in the practice of thepresent invention, one can cite those polymers and copolymers whichexhibit a second order transition temperature which is not higher thanabout 0° C., preferably not higher than about −50° C. and morepreferably not higher than about −70° C. as determined or approximatedusing conventional techniques, e.g., ASTM Test Method D-746-52 T.

By way of example rubbers can be selected from the group consisting of:

a) co- and homopolymers of C₄₋₆ conjugated diolefins,

b) copolymers comprising from 60 to 85 weight % of one or more C₄₋₆conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile and

c) copolymers comprising from 20 to 60, preferably from 40 to 50 weight% of one or more C₈₋₁₂ vinyl aromatic monomers which are unsubstitutedor substituted by a C₁₋₄ alkyl radical and from 60 to 40, preferablyfrom 60 to 50 weight % of one or more monomers selected from the groupconsisting of C₄₋₆ conjugated diolefins. The rubber may be prepared by anumber of methods, preferably by emulsion or solution polymerization.These processes are well known to those skilled in the art.

Highly preferred rubbers are alkadiene polymers. Suitable alkadienes are1,3-conjugated dienes such as butadiene, isoprene, chloroprene orpiperylene. Most preferred are homopolymers (excepting any couplingmonomers) prepared from 1,3-conjugated dienes, with such homopolymers of1,3-butadiene being especially preferred. Alkadiene copolymer rubberscontaining small amounts, for example less than 15, preferably less than10 weight percent, of other monomers such as monovinylidene aromaticscan also be employed if the rubbers meet the other qualificationsdescribed herein. The most preferred rubbers are the linear homopolymersof 1,3-butadiene which have a cis content of at least 30 percent. Therubbers suitable for the present invention can be made by anionicpolymerization or Ziegler-Natta polymerization well known to thoseskilled in the art. Such processes are described e.g. in US 2002-107339,the content of which is incorporated in the present invention.

Regarding the rubber materials suitable for use according to the presentinvention are advantageously rubbers of essentially linear type. Anessentially linear rubber is e.g. a rubber which does not contain asignificant amount of long chain branches. Such rubbers usually containless that one long chain branch per 10,000 carbon atoms on the polymerbackbone. These rubbers must have molecular-weights in the range mostsuitable for making HIPS and ABS resins. The micro-structure of thepolybutadiene rubbers can be any of the conventional types containingvarious amounts of 1,2-vinyl, 1,4-cis and 1,4-trans levels. The level ofbranching in rubbers can be determined readily by the techniquesgenerally well known to those skilled in the art as detailed in T. H.Mourey and S. T. Balke, “A Strategy for Interpreting MultidetectorSize-Exclusion Chromatography Data I: Development of a SystematicApproach,” Am. Chem. Soc. Symp. Ser., 521, 180 (1993); A. Rudin,“Measurement of Long-Chain Branch Frequency in Synthetic Polymers,” inH. G. Barth and J. W. Mays (Eds.), Modern Methods of PolymerCharacterization,” John Wiley and Sons, New York, 1991; and S. Pang andA. Rudin, “Size-Exclusion Chromatographic Assessment of Long-ChainBranch Frequency in Polyethylenes,” Am. Chem. Soc. Symp. Ser. 521, 254(1993). Another practical way to assess the rubber chain branchingdegree is from the rubber Solution Viscosity-to-Mooney ML1+4 @100° C.ratio. The lower this ratio, the higher the rubber chain branching. Therubber of essentially linear structure recommended within the frame ofthe present invention is characterized by SV-to-Mooney viscosity ratioof at least 2.8, and more preferably above 3.3.

Advantageously the Mw of the rubber ranges from 100,000 to 500,000 andpreferably from 280,000 to 360,000 g/mol. Advantageously thepolydispersity index of the rubber ranges from 2.1 to 2.5 and preferablyfrom 2.1 to 2.3. Rubber molecular-weights can be measured by theconventional size-exclusion chromatography techniques. They are hereexpressed in PS equivalents, i.e. using iso-molecular PS samples ascalibration standards.

The rubbers particularly suitable for this invention have a solutionviscosity (SV), measured at 5.43% weight in toluene or styrene, of 50 to1000 centipoises, preferably from 100 to 500 centipoises and morepreferably from 120 to 250 centipoises. The rubbers particularlysuitable for this invention also have a Mooney viscosity (ML4+1, 100°C.) of 5 to 120, preferably from 10 to 100 and more preferably from 30to 60.

The rubber is advantageously employed in amounts such that therubber-reinforced polymer product contains from about 2 to about 20percent, preferably from about 3 to about 17 percent, more preferablyabout 3 to about 15 weight percent rubber, based on the total weight ofthe rubber modified monovinylaromatic polymer.

As regards the rubber phase volume fraction (RPVF) and the swell index(SI) of the rubber particles, we need to explain the calculation ofthese parameters. In the following calculations the rubber modifiedmonovinylaromatic polymer composition is referred as HiPS for sake ofsimplicity.

The rubber phase volume fraction is a key parameter that characterizesthe rubber dispersed phase and that can be calculated from DynamicMechanical Analysis (DMA) data. The rubber phase volume refers to therubber particles or discontinuous phase, which consists of rubber,trapped polystyrene (occlusions) and grafted polymer.

The Kerner's model (Kerner, E. H., Proc. Phys. Soc. 69, 808, 1956) andits simplified versions enable to calculate the modulus of a compositemade of spherical soft particles dispersed in a rigid continuous phase.Hashin's model (Hashin, Z., ASME J. Appl. Mechanics, 29(1), 143-150,1962) is particularly well-suited for describing the rubber particledispersion in HiPS. The RPVF is calculated from Hashin's model involvingthe plateau-values of the storage modulus measured by DMA at twotemperatures:

-   -   T₁ well below the rubber glass transition temperature (T₁<Tg        rubber);    -   T₂ corresponding to a well-chosen temperature in between the        glass transition temperatures of the rubber and of polystyrene        (Tg rubber<T₂<Tg PS) at which the storage modulus G′ is steady.        T2 is preferably chosen between −30 and +30° C.

DMA measurements are carried out on compression-moulded samples, usingan ARES (TA Instruments) rheometer in the torsion geometry.

The RPVF/% rubber ratio is important in the manufacture of HIPSmaterials because it represents the “rubber utilization efficiency” ofthe process, i.e., how much rubber must be used to obtain similarproduct quality. The less rubber needed to produce a set of desiredproperties in a HIPS material, the more efficient the process.

The swell index (SI) is measured by dissolving the HiPS resin in tolueneat room temperature (25° C.). After separating the insoluble gel phaseby centrifugation, the swollen gel is weighed, dried under vacuum andthen the weight of the dry gel is obtained. The swell index is the ratioof the weight of swollen gel to dry gel, and it is a measure of thedegree of cross-linking of the rubber phase. The swell index is a commonparameter used to characterize the cross-linking degree of a rubber. Thehigher the swell index, the lower the rubber phase crosslinking level.The Gel Content (or soft insoluble rubbery fraction), defined as theratio of the weight of dry gel to the initial weight of the polymersample, gives an indication of both the rubber phase crosslinking andgrafting.

The percent rubber is the total amount of rubber in the HIPS and ismeasured by the well-known Iodine Monochloride (I—Cl) titration method.The less rubber needed to produce a set of desired properties in a HIPSmaterial, the more efficient the process.

The RPVF is at least 39%, advantageously at least 40%, preferably atleast 42% and more preferably at least 45%.

Advantageously the SI is at least 14 and preferably at least 16.

As regards the rubber particles size, as used herein, the said particlesize is the diameter of the rubber particles as measured in theresultant product, including all occlusions of matrix polymer withinrubber particles, which occlusions are generally present in the disperserubber particles of a rubber-reinforced polymer prepared using masspolymerization techniques. Regarding morphology of the rubber particlesin the different groups, as it is well known, the smaller particlestypically have a core-shell (single, major occlusion) or cellular(multiple, minor occlusions) morphology. The larger particles wouldgenerally have a cellular or similar multiple-occlusion morphology.

The volume median particle size, also referred as RPS volume, is theparticle diameter that divides the frequency distribution in volume (3rdorder moments) in half; fifty percent of the rubber volume has particleswith a larger diameter, and fifty percent of the rubber volume hasparticles with a smaller diameter. The surface median particle size,also referred as RPS surface, is the particle diameter that divides thefrequency distribution in surface (2^(nd) order moments) in half; fiftypercent of the rubber surface has particles with a larger diameter, andfifty percent of the rubber surface has particles with a smallerdiameter. The mode represents the value that occurs most frequently in adistribution. In particle size distributions, the mode is the particlediameter that occurs most frequently.

In the present invention, the RPS volume is of about 8.5 □m. The RPSvolume of about 8.5 μm means in the range 7 to 10 μm, and preferably inthe range 8.0 to 9.0 μm. The monomodal distribution means that thedistributions both in surface and volume are of the bell-shape with noshoulder nor side distributions.

In a specific embodiment the particle size can be additionallycharacterized by a RPS surface of about 6.5 μm. The RPS surface of about6.5 μm means in the range 5 to 8 μm, advantageously in the range 5.2 to7.8 μm and preferably in the range 5.5 to 7.5 μm.

The broadness of the rubber particle size distribution can be alsoroughly estimated by the RPS volume-to-RPS surface ratio, which shouldbe preferably below 2.0, more preferably below 1.5 and most preferablyequal or below 1.4.

As regards the plasticizers, represented globally by all the moleculessoluble in methanol, the weight ratio c/(a+b+c) ranges advantageouslyfrom 0 to 5% and preferably from 1 to 4.5%. The plasticizer is anycomponent that can increase the flexibility of the finished product, andreduce the melt viscosity of the bulk polymer to facilitate molding orextrusion. The plasticizer is e.g. a mineral oil or a non-mineral oil.“Mineral oil” means oil derived principally from petroleum, typicallyfrom the distillation of petroleum to produce gasoline. Typically,mineral oil is a mixture that contains a large number, e.g., hundreds,of different kinds of hydrocarbons, mainly linear and branched alkanes(n- and iso-paraffins) and cyclic paraffins, and it is liquid at ambientconditions. “Non-mineral oil” means one or more of vegetable, essential,silicone and animal oil.

“Vegetable oil” means oil derived principally from plants, particularlythe seeds and nuts of plants, and comprised largely of glycerides ofsaturated and unsaturated fatty acids, e.g., oleic, palmitic, stearicand linolenic.

“Animal oil” means oil derived principally from animals, and thoseliquid under ambient conditions include fish oils, fish-liver oils,sperm oil and the like. Animal oils typically have high saturated fattyacid content.

“Essential oil” means oil derived principally from flowers, stems andleaves. These oils are complex, volatile liquids, typically containterpenes, and contain relatively little, if any, glycerides of fattyacids.

Advantageously the plasticizer is selected among the mineral orvegetable oil compatible with the rubber. A major part of saidplasticizer migrates in the rubber and enhance the RPVF. Plasticizerswith a special structure affinity with the rubber phase and of lowvolatility are particularly recommended. By way of example, when therubber is a polybutadiene, the plasticizer can be a butene oligomer ofthe type depicted in U.S. Pat. No. 6,613,831 and also known as apolybutene oil, a crude rapeseed oil, or a refined sunflower oil,preferably mixed with a 70 to 100 cSt white mineral oil. Advantageouslythe plasticizer is introduced prior or during the polymerization.

As regards the polymerization to make the above composition, it iscarried out in a conventional manner by bulk polymerization, solutionpolymerization, or polymerization in aqueous dispersion, the rubberfirst being dissolved in the polymerizable monomer and this solutionthen being subjected to polymerization. Advantageously the process ofthe invention is carried out as a diluted bulk polymerization process.When using diluted bulk polymerization, the starting solution may bemixed with up to about ten percent (10%) by weight, based on themonovinylaromatic monomer employed, of an inert solvent so as to lowerthe polymerization bulk viscosity, to moderate polymerization heat andto improve thermal exchanges and heat homogeneity within the bulk.Suitable diluents include aromatic solvents such as ethylbenzene,toluene, xylenes, cyclohexane, and aliphatic hydrocarbon solvents, suchas dodecane, isoparaffinic cuts and white spirits exhibiting averageboiling points close to the EB and styrene b.p., and mixtures thereof.Any solvent useful to improve heat homogeneity within the bulk duringpolymerization, that can be removed after polymerization of themonovinylaromatic monomer, and that does not interfere with thepolymerization of the monovinylaromatic monomer and the optionalcomonomer, can be used with the process of the present invention.

The rubber is “dissolved” in the monovinylaromatic monomer (actually therubber is infinitely swollen with the monomer). Monovinylaromaticpolymer is initially formed from the monovinylaromatic monomer withinthe homogeneous rubber solution in monovinylaromatic monomer. At thebeginning of the polymerization the reacting solution is at a pointprior to the rubber/monovinylaromatic monomer inversion point, i.e. thepoint at which the solution being reacted goes from monovinylaromaticpolymer particles in a rubber/monovinylaromatic monomer matrix to rubberparticles in a monovinylaromatic polymer matrix. In other words when thedegree of polymerization is about equal to the weight % of rubber in thesystem it inverts e.g. the monovinylaromatic monomer/monovinylaromaticpolymer phase becomes continuous and the rubber phase becomesdiscontinuous.

In the present invention a free radical initiator and a chain transferagent are contacted with the polymerizable mixture prior to theinversion point such that when the phase inversion subsequently occurs,the molecular weight of the monovinylaromatic polymer before and duringthe phase inversion is considerably lowered as compared with what itwould be in the absence of chain transfer agent.

The purpose of said molecular weight lowering is to have simultaneouslya high viscous rubber phase and a fluid monovinylaromatic polymer phaseat inversion point. These conditions are deemed to produce large rubberparticles. Besides, they result in a much sharper increase of therubber-to-PS phase viscosity ratio around phase inversion, making thethus formed rubber particles much less sensitive to the shear rategenerated by the agitation conditions within the inversion reactor andthe subsequent reactors. In other words, the large particles thus formedin the inversion reactor are less prone to be dismantled and sheared,guaranteeing a more uniform rubber particle size distribution.

Among suitable chain transfer agents, one can cite mercaptans oralpha-methyl styrene dimer. Agents of high chain transfer activity, i.e.characterized by a transfer coefficient C_(tr)=k_(tr)/k_(p) well above1, should be preferred, as they react mostly within the reactor they areinjected in, without lowering too significantly the average molecularweight of PS in the final product As examples of mercaptans, the mostpreferred one is n-dodecyl mercaptan. Chain transfer agent is generallyemployed in an amount of from about 0.001 to about 0.5 weight percentbased on the total weight of the polymerization mixture to which it isadded. If the inversion reactor is preceded by a reactor maintainedintentionally below phase inversion, commonly known as a pre-inversionreactor, the active transfer agent is preferably introduced in thepre-inversion reactor for guaranteeing the production of a well-uniformdistribution of rubber particles in the subsequent inversion reactor.

Among the free radical initiators one can cite the peroxides. Theperoxides can be classified as grafting initiators and as non-graftinginitiators. Of course the grafting initiator doesn't make only graftingpolymerization and non-grafting initiator doesn't make only non-graftingpolymerization but the grafting initiator has an enhanced power to leadto grafting polymerization than the non-grafting initiator.

The grafting initiator is advantageously selected from the groupconsisting of 1,1-di-(t-butylperoxy)cyclohexane;1,1-di-(t-amylperoxy)cyclohexane);1,1-di-(t-butylperoxy)-3,3,5-trimethyl-cyclohexane;00-t-amyl-0-(2-ethylbexyl monoperoxy-carbonate); OO-t-butyl O-isopropylmonoperoxy-carbonate; OO-t-butyl-0-(2-ethylhexyl)monoperoxy-carbonate;butyl 4,4-di(t-butylperoxy)valerate; Ethyl3,3-Di-(t-butylperoxy)butyrate; and mixtures thereof. It is typically aperoxydic molecule releasing by thermal decomposition ter-butoxylradicals that are prone to undergo H-abstracting reactions. Thenon-grafting initiator is advantageously selected from the groupconsisting of 2,2′-azobis(isobutyronitrile),2,2′-azobis(2-methylbutyronitrile), lauroyl peroxide, decanoyl peroxide,and mixtures thereof. From about 50 to about 2000, preferably from about100 to about 1500, weight parts of the initiator are employed permillion weight parts of the polymerizable mixture.

Monovinylaromatic monomer is polymerized around and within the rubberparticles, which leads to monovinylaromatic polymer inclusions in therubber particles. A portion of the monovinylaromatic monomer ispolymerized by grafting on the rubber, another portion ishomopolymerized, said portion is referred to as a “non-grafting”polymerization. A part of the monovinylaromatic monomer may be replacedby unsaturated copolymerizable monomers as explained above. The samemechanism of “grafting” and “non-grafting” occurs with the comonomer,which means one portion of the monovinylaromatic monomer and of thecomonomer are polymerized by grafting on the rubber and another portionof the monovinylaromatic monomer and of the comonomer are copolymerized.

The high impact monovinylaromatic polymers of the present invention canbe prepared using additives. Exemplary additives include fillers such astalc, organoclays (clays wetted by an organic compatibilizer), antioxidants (e.g., alkylated phenols such as di-tert-butyl-p-cresol orphosphates such as tri-nonyl phenyl phosphate), UV stabilizers,lubricants, mold release agents e.g., zinc stearate; PE waxes, pigmentsand the like. Any additive known to be useful in preparing high impactmonovinylaromatic polymers to those of ordinary skill in the art ofpreparing such polymers can be used with the present invention. Theseadditives, if any, are added to the reaction mixture where appropriateincluding before, during or after polymerization.

Although the resulting polymeric materials are particularly suitable asliners in a refrigerator or a freezer, they can be of interest invarious uses including (non exhaustive list) TV front and back covers,households, electronic and electric appliances, dairy cups, foodpackagings, insulation foams, etc. . . . . All these apparatus can bemade by the usual technology of the thermoplastics. Refrigerator linersare commonly manufactured industrially using extruded sheets of HIPS.The sheets are thermoformed into the desired shape and size by firstheating them to bring the polymer above its glass transitiontemperature. The softened polymer is then pressed into a predeterminedshape of a door or inner liner.

The present invention is also a process for preparing a high impactmonovinylaromatic polymer comprising:

a) forming a polymerizable mixture comprising at least onemonovinylaromatic monomer and optionally one or more comonomers,

b) dissolving at least a viscous rubber of essentially linear structurein said polymerizable mixture to form a rubber containing polymerizablesolution,

c) contacting a free radical initiator and a chain transfer agent withthe polymerizable mixture at conditions whereby phase inversionsubsequently occurs, wherein the chain transfer agent is capable toproduce an increase of the rubber to PS phase viscosity ratio within theinversion reactor,d) continuing the polymerization of the solution obtained at step c),optionally in the presence of a free radical initiator, until amonovinylaromatic polymer matrix having rubber particles dispersedtherein is obtained.

Optionally there is an additional step for degassing the product of stepd) to separate the optional unpolymerized monomers and comonomers,optional diluents and recovering the high impact monovinylaromaticpolymer.

Advantageously the chain transfer agent makes the rubber particles lesssensitive to the shear rate within the inversion reactor. This resultsin a homogeneous distribution of large rubber particles.

The advantageous rubber of essentially linear structure within the frameof the present invention is characterized by SV-to-Mooney ratio of atleast 2.8, and more preferably above 3.3.

Advantageously the Mw of the rubber ranges from 100,000 to 500,000 andpreferably from 280,000 to 360,000 g/mol. Advantageously thepolydispersity index of the rubber ranges from 2.1 to 2.5 and preferablyfrom 2.1 to 2.3. Rubber molecular-weights can be measured by theconventional size-exclusion chromatography techniques. They are hereexpressed in PS equivalents, i.e. using iso-molecular PS samples ascalibration standards.

The rubbers particularly suitable for this invention have a solutionviscosity (SV), measured at 5.43% weight in toluene or styrene, of 50 to1000 centipoises, preferably from 100 to 500 centipoises and morepreferably from 120 to 250 centipoises. The rubbers particularlysuitable for this invention also have a Mooney viscosity (ML4+1, 100°C.) of 5 to 120, preferably from 10 to 100 and more preferably from 30to 60.

The rubber is advantageously employed in amounts such that therubber-reinforced polymer product contains from about 2 to about 20percent, preferably from about 3 to about 17 percent, more preferablyabout 3 to about 15 weight percent rubber, based on the total weight ofthe rubber modified monovinylaromatic polymer.

All the operating conditions relating to the process described above andconcerning the process to make the liners for refrigerators and freezersare available in this process.

EXAMPLES

The examples which follow illustrate the invention. They should not beconsidered as limitations of the present invention. Parts andpercentages are by weight, unless stated otherwise. The products aretested bu the following methods:

-   1. Molecular-weights are measured by size exclusion chromatography,    using iso-molecular PS standards from Polymer Laboratories for the    calibration.-   2. Rubber particle sizes are measured with a laser granulometer.-   3. The swell index, when mentioned, is calculated from the wet    weight and dry weight obtained by ultra-centrifugating a solution of    HiPS in toluene, according to the method described above.-   4. The rubber phase volume fraction is calculated from Hashin's    model and DMA results, using the Equation (1) given above.-   5. Tensile properties are measured according to ISO 527-2.-   6. Flexural properties are measured according to ISO 178.-   7. The notched Izod values are obtained according to ISO 180.-   8. Vicat 50N is measured according to ISO 306.-   9. The melt-flow indices are measured following ISO 1133 (5 kg    loading, 200° C.).-   10. Resistance to stress-cracking agents: This test was derived from    ISO 4599.    Five (5) injected dumbbell shipped halters (10*4*149.3 mm, ISO 527-2    A1) are exposed to fatty esters (e.g. olive oil) and submitted to a    steady deformation for 1 day, 3 days, 4 days and 7 days,    respectively. The deformation consists in bending the halters by    setting a distance between clamps of 143 mm, which corresponds to a    c.a. 20 MPa surface stress (see FIG. 1). After being removed from    their support, the bended halters are let relax for 2 hrs under a 5    kg load prior to being submitted to tensile test measurements at a    50 mm/min rate. An extensiometer is used to record the displacement    and the elongation at break with a good accuracy. Five (5)    non-deformed and non-oil-exposed halters are used as references and    submitted solely to tensile test measurements. The stress-cracking    resistance is expressed in % as the averaged elong. @ brk of the    oil-exposed & deformed specimens divided by the averaged elong. @    brk of the reference specimens. See FIG. 1;

A variant of this test consists in following up the drop of theelongation properties—the non-exposed and non-deformed samples beingstill taken as references—after a given time.

Example 1

A 5 liter continuously stirred tank reactor (CSTR) used as an inversionreactor is continuously fed with a 1.5 liter/hr flow of a rubbersolution consisting of:

-   -   (i) 7.7% of a 170 cps LiBR, commercialized by Lanxess under the        tradename of Buna CB 528 T (aver. Mw=290,000 g/mol;        SV-to-Mooney=3.1);    -   (ii) 2.5% wt of 70 cSt white mineral oil;    -   (iii) 6% wt Ethylbenzene;

This feed composition is polymerized within this inversion reactor at atemperature of 135° C. in the presence of 60 ppm of commercialter-butylperoxy-isopropylcarbonate (Akzo Trigonox BPIC-C75, injected asit is) and of 60 ppm of commercial NDM (n-dodecyl mercaptan) fromArkema. The average residence time is of c.a. 90 min. The agitationspeed in the reactor was adjusted so as to maintain a max shear rate atthe tip of the blade of 48 s⁻¹. The bulk collected at the outlet of theinversion reactor and characterized by a solid content of c.a. 35% wasfurther polymerized in a 20 liter batch reactor with a temperatureprofile of 140° C. (30 min) and 150° C. up to 60% solids. The resultingproduct was finally degassed in a single flash devolatilizer at 230° C.under 25 mbars prior to being pelletized. The resulting characteristicsand properties of the final products are listed in Table 1.

Example 2

The same procedure is applied than in example 1, except for the rubberwhich is replaced by a 170 cps NdBR, commercialized by Lanxess under thetradename of Buna CB 728 T aver. Mw=455,000 g/mol; SV-to-Mooney=3.9).The use of this substantially more linear rubber of high-cismicrostructure results in better cold Izod and stress-crackingresistance properties (see Table 1), fatty cream being here taken as theaggressive agent.

TABLE 1 Product characteristics & properties Example 1 Example 2 RPS,D50 vol (microns) 7.6 8.8 Rubber content (%) 10.5 10.3 RPVF (%) 39.740.5 Mw (g/mol) 205,000 207,500 MFI (g/10 min) 2.8 2.6 Flex. Modulus(MPa) 1791 1858 Notched Izod (kJ/m²) at 12.1 14.4 room temp. Elong. @Brk (%) 55 52 Izod loss @ −30° C. (%) 37 23 ESCR with fatty cream 28 6(30% fat) for 6 hrs - Elong. @ Brk. Loss (%)

Examples 3-5

A rubber solution feed consisting of 8.5% of commercial 170 cps LiBRIntene 50 AF from Polimeri Europa and 3% of 70 cSt white mineral oilcontinuously feeds a series of polymerization reactors. The rubbersolution is mixed with a recycle flow made of c.a. 58% styrene monomerand 42% diluent (éthylbenzène mainly). The global flowrate within thereactor train is of 40 kg/hr.

The feed solution is preheated at 90° C. in a tubular preheater. LuperoxTBIC-M75 (commercial organic peroxide) and NDM, both from Arkema, areco-injected in a first CSTR-type reactor, R-201L, the temperature ofwhich is set at 110° C. and which has an average residence time of 45min. The solid content of this reactor is kept within the 12-14% solidsrange, i.e. well below phase inversion. The bulk solution therein ismaintained under a high shear rate to limit the risks of formation ofrubbery gels. Phase inversion proceeds under various conditions in thesubsequent CSTR-type reactor R-202L, characterized by an averageresidence time of 110 min. and smooth agitation conditions (theagitation speed is set so as to maintain a max shear rate at the tip ofthe blade of 11 s⁻¹). The polymerization is then finished in a series ofplug-flow type reactors, before the product is degassed in a series oftwo flash-devolatilizers and pelletized.

In Example 3, the TBIC dosage and the reactor & DV settings are adjustedso as to produce a standard HiPS suitable for conventionalextrusion-thermoforming applications. No NDM is injected in thereactors.

In Example 4, the TBIC & NDM dosages as well as the reactor settings areadjusted so as to produce a HiPS grade of improved stress-crackingresistance, as described in the embodiments of the present invention.

The ESCR grade is further improved in Example 5 with refineddevolatilization conditions.

The production conditions of the products of Examples 3-5 are describedin Table 2 bellow, whereas the related product characteristics &properties are listed in Table 3.

FIG. 2 below shows the RPS distributions in volume of the products,whereas FIG. 3 gives a representation of their ESCR resistance withtime, olive oil being taken as the stress-cracking agent. In table 3 RPSdispersion V/S means RPS volume-to-RPS surface ratio.

TABLE 2 Operating Conditions Example 3 Example 4 Example 5 TemperatureR-201 L (° C.) 110 110 110 TBIC-M75 peroxide injected 150 150 150 inR-201 L (ppm) Solid content in R-201 L (%) 12.1 13.7 13.4 TemperatureR-202 L (° C.) 124 129 129 NDM injected in R-202L (ppm) 0 150 150 Solidcontent in R-202 L (%) 28.7 35.0 34.9 Temperature profile 141/158141/156 141/156 in the plug-flow finishing reactors (° C.) Solid contentat the outlet 70.1 74.3 74.5 of the last reactor (%) DV1 Preheater OIL255 255 246 Temperature (° C.) DV1 Polymer Temp/° C. 241 242 229 DV2Polymer Temp/(° C.) 240 240 230 Vacuum @ 1st Devol (mmHg) 19 22 28Vacuum @ 2nd Devol (mmHg) 6 6 6

TABLE 3 Iot nr Example 3 Example 4 Example 5 Mw (kDa) 167 156 155 % Pbu(%) 8.5 8.5 8.5 particles S/V (μm) 3.3/3.9 5.6/8.0 5.9/8.3 RPSdispersion, V/S 1.2 1.4 1.4 RPVF (%) 35.7 39.4 42.2 Gel content (%) 29.731.4 29.5 Swell Index 13.5 13.9 16.4 MFI (g/10′) 3.9 4.5 4.8 Vicat 50N(° C.) 88.9 87.8 86.3 Flexural Modulus (Prat) (MPa) 1610 1520 1550Notched Izod (KJ/m2) 8.3 8.5 9.5

The invention claimed is:
 1. A refrigerator interior liner comprising: arubber modified monovinylaromatic polymer composition, wherein therubber modified monovinylaromatic polymer composition comprises: a) amonovinylaromatic polymer matrix having an average molecular weight inweight Mw above 150,000 g/mol; and b) rubber particles dispersed in themonovinylaromatic polymer matrix, wherein the rubber particles have: avolume median particle size (RPS volume) ranging from 7 μm to 10 μm; amonomodal distribution; a ratio of RPS volume to surface median particlesize (RPS surface) that is below 2; a crosslinking of the rubberexpressed as a swell index (SI) above 13.8; and a rubber phase volumefraction (RPVF) of at least 39%.
 2. The refrigerator interior liner ofclaim 1, wherein the rubber particles have an RPS surface of from 5 to 8μm.
 3. The refrigerator interior liner of claim 1, wherein the rubberhas a solution viscosity-to-Mooney viscosity ratio of at least 2.8. 4.The refrigerator interior liner of claim 1, wherein the rubber has anaverage molecular weight in weight Mw ranging from 100,000 g/mol to500,000 g/mol.
 5. The refrigerator interior liner of claim 1, whereinthe rubber has a polydispersity index ranging from 2.1 to 2.5.
 6. Therefrigerator interior liner of claim 1, wherein the rubber has asolution viscosity (SV), measured at 5.43% weight in toluene, of 50 to1000 centipoises.
 7. The refrigerator interior liner of claim 1, whereinthe rubber has a Mooney viscosity (ML4+1, 100° C.) of 5 to
 120. 8. Therefrigerator interior liner of claim 1, wherein the monovinylaromaticpolymer has a polydispersity ranging from 2.2 to
 3. 9. The refrigeratorinterior liner of claim 1, wherein the rubber modified monovinylaromaticpolymer composition comprises from 2 to 20 weight percent of the rubberparticles.
 10. The refrigerator interior liner of claim 1, wherein therubber modified monovinylaromatic polymer composition has a melt flowindex ranging from 2 to 10 g/10 min, measured following ISO 1133 at 200°C. and a load of 5 kg.
 11. A rubber modified monovinylaromatic polymercomposition comprising: a) a monovinylaromatic polymer matrix having anaverage molecular weight in weight Mw above 150,000 g/mol; and b) rubberparticles dispersed in the monovinylaromatic polymer matrix, wherein therubber particles have: a volume median particle size (RPS volume)ranging from 7 μm to 10 μm; a monomodal distribution; a ratio of RPSvolume to surface median particle size (RPS surface) that is below 2; acrosslinking of the rubber expressed as a swell index (SI) above 13.8;and a rubber phase volume fraction (RPVF) of at least 39%.
 12. Therubber modified monovinylaromatic polymer composition of claim 11,wherein the rubber particles have an RPS surface of from 5 to 8 μm. 13.The rubber modified monovinylaromatic polymer composition of claim 11,wherein the rubber has a solution viscosity-to-Mooney viscosity ratio ofat least 2.8.
 14. The rubber modified monovinylaromatic polymercomposition of claim 11, wherein the rubber has an average molecularweight in weight Mw ranging from 100,000 g/mol to 500,000 g/mol.
 15. Therubber modified monovinylaromatic polymer composition of claim 11,wherein the rubber has a polydispersity index ranging from 2.1 to 2.5.16. The rubber modified monovinylaromatic polymer composition of claim11, wherein the rubber has a solution viscosity (SV), measured at 5.43%weight in toluene, of 50 to 1000 centipoises.
 17. The rubber modifiedmonovinylaromatic polymer composition of claim 11, wherein the rubberhas a Mooney viscosity (ML4+1, 100° C.) of 5 to
 120. 18. The rubbermodified monovinylaromatic polymer composition of claim 11, wherein themonovinylaromatic polymer has a polydispersity ranging from 2.2 to 3.19. The rubber modified monovinylaromatic polymer composition of claim11, wherein the rubber modified monovinylaromatic polymer compositioncomprises from 2 to 20 weight percent of the rubber particles.
 20. Therubber modified monovinylaromatic polymer composition of claim 11,wherein the rubber modified monovinylaromatic polymer composition has amelt flow index ranging from 2 to 10 g/10 min, measured following ISO1133 at 200° C. and a load of 5 kg.